Photonic crystals (PCs) are materials having a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987), giving rise to a photonic band gap or stop gap, in which electromagnetic waves within a certain stop band wavelength range are totally reflected. The wavelengths of the stop band are dependent on the distance between the periodic modulations in the crystal. The reflected stop band wavelengths appear in the reflectance spectrum as a distinct reflectance peak known as a Bragg peak. The crystal may have a one-, two-, or three-dimensional periodic structure.
Because of the sensitivity of a PC, slight changes in the refractive index or lattice spacing results in detectable changes in the reflected light. This may be particularly useful where the reflected light is in the visible range, allowing for changes in color if the refractive index or lattice spacing is modulated. By incorporating polymers into PC materials, these materials may be made responsive to external stimuli, such as mechanical forces. An example of such an application is described by Arsenault et al. in PCT Patent Application No. 2008/098339, which is herein incorporated by reference in its entirety.
Perishable products such as foodstuffs, alcoholic beverages, pharmaceuticals, cosmetics, biological materials, chemical substances, live tissue samples, photographic materials and vaccines typically require storage with certain temperature ranges if stored for certain lengths of time. They could suffer loss of quality when they are exposed, even briefly, to temperatures outside their respective ranges or being stored for longer than their intended shelf lives. Temperature sensitive devices that change color in response to cumulative exposure to time and unfavorable temperatures have been used for monitoring the storage and handling of perishable products. Such indicator systems are used to provide visual signals to when a product has reached a point of unsafe or unappealing quality or the end of its useful and/or specified shelf life, for example due to excessive exposure to temperature above a certain threshold for a given amount of time.
Conversely, certain items, such as meats, may require a given minimum threshold temperature to be reached before the item can be safely, effectively, or reliably used. An indicator system could then show that such a threshold temperature has indeed been reached.